Use of water-soluble copolymers of monoethylenically unsaturated carboxylic acids and vinylimidazoles or derivatives thereof for water treatment

ABSTRACT

Water-soluble copolymers which contain, polymerized in, as characteristic monomers 
     (a) 99 to 50% by weight of monoethylenically unsaturated carboxylic acids with 3 to 8 carbon atoms or their salts and 
     (b) 1 to 50% by weight of N-vinylimidazole or substituted N-vinylimidazoles, their salts or products of quaternization and which have K values of from 10 to 50 are used for water treatment to reduce deposits of scale and sludge in water-conveying systems.

The present invention relates to the use of water-soluble copolymers ofmonoethylenically unsaturated carboxylic acids and vinylimidazoles forwater treatment and scale inhibition.

US-B 3,810,834 discloses the use of hydrolyzed polymaleic anhydrideswhich have a molecular weight of from 300 to 5,000 before thehydrolysis, or the water-soluble salts thereof, for treating water inorder substantially to reduce or prevent scale formation. The polymerssuitable for this purpose are prepared by polymerization of maleicanhydride in toluene, using benzoyl peroxide, and subsequent hydrolysisof the resulting polymaleic anhydride. Since maleic anhydride does notpolymerize completely and it is difficult to remove unpolymerized maleicanhydride from the polymer, the polymaleic acids contain considerableamounts of maleic acid.

US-B 3,755,264 discloses low molecular weight copolymers which contain85 to 99 mol % maleic anhydride and, polymerized in to make up to 100mol %, acrylic acid, vinyl acetate, styrene or mixtures thereof. Thecopolymers are prepared by copolymerization of maleic anhydride with thesaid monomers in dry organic solvents at from 100° to 145° C. in thepresence of peroxides. Examples of suitable peroxides are di-tert-butylperoxide, acetyl peroxide, dicumyl peroxide, diisopropyl percarbonateand, in particular, benzoyl peroxide. The anhydride copolymer cansubsequently be hydrolyzed to the acid or converted into the salts. Thewater-soluble copolymers are used to prevent deposition of scale. Theproducts obtainable by this process contain a very large amount ofunpolymerized maleic anhydride.

The use of low molecular weight polymers of acrylic acid for watertreatment or scale inhibition is disclosed, for example, in US-B3,904,522 and US-B 3,514,376. US-B 3,709,816 discloses that copolymerscontaining acrylamidopropanesulfonic acid are suitable for watertreatment. Suitable examples are copolymers with2-acrylamidopropanesulfonic acid and acrylamide which is partiallyhydrolyzed. The disadvantage of these is that residues of acrylamidemonomer are unavoidable in the polymers, which means that they can berecommended for use only with great restrictions. On the other hand,acrylic acid homopolymers have a satisfactory effect only on types ofdeposits which are relatively easy to reduce, such as calcium carbonate.

The object of the present invention is to provide polymers for watertreatment which equal or even exceed the effectiveness of the polymersbased on acrylic acid which have been used hitherto, and which are alsomore soluble at high calcium ion concentrations than the polyacrylateshitherto used. In particular, the intention is effectively to reduceespecially difficult problems of deposits in water-conveying systems,such as the formation of calcium phosphate and of silicate deposits.

We have achieved this object according to the invention by usingwater-soluble copolymers which contain, polymerized in, ascharacteristic monomers

(a) 99 to 50% by weight of monoethylenically unsaturated carboxylicacids with 3 to 8 carbon atoms or their salts and

(b) 1 to 50% by weight of monomers of the formula ##STR1## where R¹, R²and R³ are H, C₁ --C₄ --alkyl, phenyl or benzyl, their salts or productsof quaternization and which have K values of from 10 to 50 (determinedin 1% strength aqueous solution at pH 7 and 25° C. by the method of H.Fikentscher) for water treatment.

Copolymers of the type described above are known in principle. Thus, forexample, US-B 3,634,366 describes a copolymer of acrylic acid and1-vinyl-2-methylimidazole. The copolymers are prepared bycopolymerization of the monomers in the presence of polymerizationinitiators. The copolymers to be used according to the invention containas characteristic monomers of group (a) monoethylenically unsaturatedcarboxylic acids with 3 to 8 carbon atoms or their salts. Examples ofthese monomers are acrylic acid, methacrylic acid, dimethylacrylic acid,ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid,allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid,mesaconic acid and itaconic acid. Monomers from this group which arepreferably used for the preparation of the copolymers to be usedaccording to the invention are acrylic acid, methacryclic acid or maleicacid and mixtures of the said carboxylic acids, especially mixtures ofacrylic acid and maleic acid. These monomers can be present in thecopolymers either in the form of the free acids or in a form which ispartially or completely neutralized. These monomers are neutralizedwhere appropriate with alkali metal bases, ammonia or amines. The basesof particular practical importance are solutions of sodium or potassiumhydroxide and ammonia. It is equally possible to carry out theneutralization with amines such as ethanolamine, diethanolamine ortriethanolamine. The monomers of group (a) contribute 99 to 50,preferably 95 to 70%, by weight of the structure of the copolymers.

The copolymers contain, polymerized in, as characteristic monomers ofgroup (b) compounds of the formula ##STR2## where R¹, R² and R³ are H,C₁ --C₄ --alkyl, phenyl and benzyl and the salts or products ofquaternization of the compounds of the formula I. To form salts, themonomers of the formula I are neutralized with acids, e.g. hydrochloricacid, sulfuric acid, nitric acid, phosphoric acid, formic acid, aceticacid, propionic acid, amidosulfonic acid or p-toluenesulfonic acid. Thequaternization of the compounds of the formula I can be carried out withconventional quaternizing agents, e.g. dimethyl sulfate, diethylsulfate, methyl chloride, ethyl chloride, butyl chloride, ethylhexylchloride, n-dodecyl chloride and benzyl chloride. It is also possible tocarry out subsequent quaternization with conventional quaternizingagents of copolymers which contain a compound of the formula Ipolymerized in.

Examples of suitable compounds of the formula I are 1-vinylimidazole,1-vinyl-2-methylimidazole, 1-vinyl-4-methylimidazole,1-vinyl-5-methylimidazole, 1-vinyl-2-ethylimidazole,1-vinyl-2-propylimidazole, 1-vinyl-2-phenylimidazole,1-vinyl-4,5-benzimidazole and 1-vinyl-2-benzimidazole [sic]. To preparethe copolymers, the said compounds can be subjected to thecopolymerization either alone or mixed with one another. The monomerfrom this group which is preferably used is 1-vinylimidazole. Themonomers of group (b) contribute from 1 to 50, preferably 5 to 30%, byweight of the structure of the copolymers.

The copolymers can be modified by containing, polymerized in, as afurther group of monomers (c) other monoethylenically unsaturatedmonomers which can be copolymerized with monomers (a) and (b). Theamount of these monomers polymerized into the copolymers of (a) and (b)is only such that the copolymers are still soluble in water. The amountof monomers (c) can therefore vary within a wide range. Where themonomers ((c) are polymerized into the copolymers for modification, theycontribute up to 20% by weight of the structure of the copolymers. It ispossible to use for the modification, for example, esters, amides andnitriles of the carboxylic acids mentioned under (a). Examples ofpreferred compounds of this type are methyl acrylate, ethyl acrylate,methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate,hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethylmethacrylate, hydroxypropyl methacrylate, monomethyl maleate, dimethylmaleate, monoethyl maleate, diethyl maleate, acrylamide, methacrylamide,N-dimethylacrylamide [sic], N-tert-butylacrylamide,dimethylaminopropylmethacrylamide, acrylamidoglycolic acid,acrylonitrile and methacrylonitrile. Also suitable as monomers of group(c) are those containing sulfo groups, e.g. vinylsulfonic acid,allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid,3-sulfopropyl acrylate, 3-sulfopropyl methacrylate andacrylamidomethylpropanesulfonic acid, and those containing phosphonogroups such as vinylphosphonate [sic], allylphosphonate [sic] andacrylamidomethanepropanephosphonic [sic] acid. Further suitable monomersof group (c) are N-vinylpyrrolidone, N-vinylcaprolactam,N-vinylformamide, N-vinyl-N-methylformamide, N-vinylacetamide,N-vinyl-N-methylacetamide, N-vinyl-2-methylimidazoline,diallyldimethylammonium chloride, vinyl acetate and vinyl propionate. Itis of course also possible to use mixtures of the said monomers of group(c), e.g. ethyl acrylate and vinyl acetate or acrylamide andhydroxyethyl acrylate. Monomers of group (c) particularly suitable forthe modification of the copolymers of (a) and (b) are vinylsulfonicacid, acrylamidomethanepropanesulfonic [sic] acid, N-vinylpyrrolidone,N-vinylformamide, diallyldimethylammonium chloride and vinyl acetate.Where the monomers of group (c) are polymerized in the copolymers of (a)and (b) for modification, they are preferably present in amounts of upto 10% by weight.

The use of copolymers which contain, polymerized in,

(a) 95 to 70% by weight of acrylic acid, methacrylic acid, maleic acidor mixtures of the said carboxylic acids and

(b) 5 to 30% by weight of N-vinylimidazole and

(c) 0 to 10% by weight of vinylsulfonic acid,acrylamidomethylpropanesulfonic acid,N-vinylpyrrolidone,N-vinylformamide, diallyldimethylammonium chloride or hydroxypropylacrylate

is particularly preferred. The copolymers are preferably used incompletely or partly neutralized form. The copolymers have K values offrom 10 to 50, preferably 15 to 40 (determined on 1% by weight solutionsof the sodium salts of the copolymers at pH 7 and 25° C. by the methodof H. Fikentscher).

The copolymers can be prepared by all conventional continuous orbatchwise processes of bulk, precipitation, suspension and solutionpolymerization in the presence of polymerization initiators which formradicals under the polymerization conditions, e.g. inorganic and organicperoxides, persulfates, azo compounds and redox catalysts.

Suitable and preferred radical initiators are all those compounds whichhave a half-life of less than 3 hours at the chosen polymerizationtemperature. If the polymerization is started at low temperature andcompleted at higher temperature, it is expedient to use at least 2initiators which decompose at different temperatures, i.e. an initiatorwhich decomposes at low temperature to start the polymerization and thenan initiator which decomposes at higher temperature for completion ofthe main polymerization. It is possible to employ initiators which aresoluble or insoluble in water or mixtures of the two types. Initiatorswhich are insoluble in water are soluble in the organic phase. Examplesof initiators which can be used for the following temperature ranges areindicated:

Temperature: 40° to 60° C.: acetyl cyclohexanesulfonyl peroxide,diacetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate,di-2-ethylhexyl peroxydicarbonate, tert-butyl perneodecanoate,2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2'-azobis(2-methyl-N-phenylpropionamidine) dihydrochloride and2,2'-azobis(2-methylpropionamidine) dihydrochloride

Temperature: 60° to 80° C.: tert-butyl perpivalate, dioctanoyl peroxide,dilauroyl peroxide and 2,2'-azobis(2,4-dimethylvaleronitrile)

Temperature: 80° to 100° C.: dibenzoyl peroxide, tert-butylper-2-ethylhexanoate, tert-butyl permaleate,2,2'-azobis(isobutyronitrile) and dimethyl 2,2'-azobisisobutyrate

Temperature: 100° to 120° C.: bis(tert-butylperoxide)cyclohexane [sic],tert-butyl peroxyisopropyl [sic] carbonate and tert-butyl peracetate

Temperature: 120° to 140° C.: 2,2-bis(tert-butylperoxy)butane, dicumylperoxide, di-tert-amyl peroxide and di-tert-butyl peroxide

Temperature: >140° C.: p-methane [sic] hydroperoxide, penane [sic]hydroperoxide, cumene hydroperoxide and tert-butyl hydroperoxide

Additional use of salts or complexes of heavy metals, e.g. copper,cobalt, manganese, iron, nickel and chromium salts or organic compoundssuch as benzoin, dimethylaniline or ascorbic acid together with at leastone of the abovementioned initiators may reduce the half-lives of theradical initiators mentioned. Thus, for example, tert-butylhydroperoxide can be activated by addition of 5 ppm copper(II)acetylacetonate so that polymerization can be carried out at only 100°C. The reducing component of redox catalysts can also be formed, forexample, by compounds such as sodium sulfite, sodium bisulfite, sodiumformaldehyde bisulfite and hydrazine. Based on the monomers used in thepolymerization, 0.01 to 20, preferably 0.05 to 10, % by weight of apolymerization initiator or a mixture of several polymerizationinitiators is used. 0.01 to 5% of the reducing compounds are added asthe redox components. Heavy metals are used in the range from 0.1 to 100ppm, preferably 0.5 to 10 ppm. It is often advantageous to employ acombination of peroxide, reducing agent and heavy metal as redoxcatalyst. Copolymerization of the essential monomers (a) and (b) canalso be carried out by exposure to ultraviolet radiation in the presenceor absence of UV initiators. The conventional photoinitiators andsensitizers suitable for polymerization by exposure to UV radiation areused for this purpose. Examples of these are compounds such as benzoinand benzoin ethers, α-substituted benzoin compounds such asα-methylolbenzoin and α-methylolbenzoin ethers, α-methylbenzoin orα-phenylbenzoin. It is also possible to use triplet sensitizers such asbenzyl [sic] diketals. Examples of sources of UV radiation used are,besides high-energy UV lamps such as carbon arc lamps, mercury vaporlamps or xenon lamps, also low-UV light sources such as fluorescenttubes with a high blue content.

In order to prepare polymers with a low K value, the polymerization isexpediently carried out in the presence of regulators. Examples ofsuitable regulators are mercapto compounds such as mercaptoethanol,mercaptopropanol, mercaptobutanol, mercaptoacetic acid,mercaptopropionic acid, butyl mercaptan and dodecyl mercaptan. Alsosuitable as regulators are allyl compounds such as allyl alcohol,aldehydes such as formaldehyde, acetaldehyde, propionaldehyde,n-butyraldehyde and isobutraldehyde, formic acid, propionic acid,hypophosphorous acid and phosphorous acid. If the polymerization iscarried out in the presence of regulators, 0.05 to 20% by weight ofthese are required, based on the monomers used in the polymerization. Inorder to prepare copolymers with K values from 30 to 50, it may beexpedient to carry out the copolymerization also in the presence ofmonomers which have at least two ethylenically unsaturated unconjugateddouble bonds in the molecule. This group of monomers comprises, forexample, croslinkers such as methylenebisacrylamide, esters of acrylicacid and methacrylic acid with polyhydric alcohols, e.g. glycoldiacrylate, glycerol triacrylate, glycol dimethacrylate and glyceroltrimethacrylate, and polyols esterified at least twice with acrylic acidor methacrylic acid, such as pentaerythritol and glucose. Other suitablecrosslinkers are divinylbenzene, divinyldioxane, pentaerythritoltriallyl ether and pentaallylsucrose. If crosslinkers are used in thecopolymerization, the amount thereof is up to 5% by weight based on thetotal monomers.

In bulk polymerization, the monomers are heated together with theradical initiators, it usually being necessary to heat the reactants toabove the softening point in order to keep the mass fluid. Preparationis expediently carried out continuously in order to be able reliably todispel the high heat of polymerization. This usually results in polymerswith K values in the range from 10 to about 30. To prepare copolymerswith K values of more than 30 to 50, precipitation or suspensionpolymerization can be used. In precipitation polymerization, themonomers are soluble in the diluent, and the copolymers which are formedare insoluble and therefore precipitate out. In suspensionpolymerization, monomers and polymers are insoluble in the diluent. Inorder to prevent the copolymer particles sticking together, thecopolymerization is expediently carried out in the presence ofprotective colloids. After completion of the copolymerization, thecopolymers can be isolated in solid form by filtration and drying. Thepreferred polymerization method is solution polymerization in whichmonomers and copolymers are dissolved in the solid. Particularlysuitable solvents for solution polymerization are water, secondaryalcohols and mixtures of water and secondary alcohols. Where water isused as solvent, the polymerization must be carried out in the presenceof regulators otherwise the resulting copolymers have too high a Kvalue. On the other hand, if the monomers are polymerized in secondaryalcohols it is possible to dispense with the addition of regulatorsbecause it is known that secondary alcohols act as regulators.

The polymerization in the said process is carried out in such a way thatthe polymer concentration is from 5 to 80, preferably 10 to 60, % byweight. Suitable temperatures are from 20° to 250°, preferably 40° to180° C. Very particularly preferred temperatures for thecopolymerization in practice are from 60° to 130° C. Where thetemperature is above the boiling point of the solvent or mixturethereof, the copolymerization is carried out under elevated pressure.When the copolymerization is carried out in an organic solvent, afterits completion the reaction mixture is neutralized where appropriate andthen subjected to steam distillation to remove the organic solvent. Itis of course also possible to remove the organic solvent from thereaction mixture by distillation and then to add water in order toobtain a copolymer solution. The copolymer is neutralized if desired.The aqueous copolymer solutions obtained in this way can be useddirectly for water treatment to reduce the deposition of scale andsludge in water-conveying systems. It is possible to combine thepolymers according to the invention with other dispersing agents such asphosphonates, phosphonoalkanecarboxylic acids etc.

The mode of action of the copolymers to prevent deposits in watertreatment comprises prevention of the formation of crystals of the saltsresponsible for hardness, such as calcium carbonate, magnesium oxide,magnesium carbonate, calcium, barium or strontium sulfate, calciumphosphate (apatite) and the like at a dose which is less thanstoichiometric, or influencing the formation of these precipitates insuch a way that no hard deposits are produced and only sediments whichare finely divided in the water and can easily be flushed out areformed. In this way the surfaces of, for example, heat exchangers, pipesor pump components are kept free of deposits and their proneness tocorrosion is reduced. In particular, the danger of pitting andperforation under these deposits is reduced. In addition, the growth ofmicroorganisms on these metal surfaces is impeded. Prevention ofdeposits in this way is able to increase the useful life of such systemsand reduce considerably stoppages for cleaning components. The amountsof the antideposit agents required for this purpose are from 0.1 to 100,preferably 0.5 to 25, ppm based on the amount of water in each case. Thewater-conveying systems are, for example, open or closed coolingcirculations, for example of power stations or chemical plants, such asreactors, distillers and similar components, where heat must bedispelled. The antideposit agents can also be used in boiler water andvaporizers, preferably at water temperatures below 150° C. A preferreduse of the antideposit agents to be used according to the invention isthe desalination of seawater and brackish water by distillation ormembrane processes, such as reverse osmosis or electrodialysis. Thus,for example, in the multistage flash evaporation (MSF) process fordesalination of seawater, concentrated seawater is circulated atelevated temperature. In this case the antideposit agents effectivelysuppress deposits of, for example, brucite and their adherence tocomponents of the system.

In membrane processes, the damage to the membranes from crystallizationof hardness-producing salts can be effectively prevented. Theseantideposit agents thus make possible higher concentration factors,improved yields of pure water and longer useful lives of the membranes.Another use of the antideposit agents is, for example, in theevaporation of syrups from cane or beet sugar. In contrast to the usesdescribed above, in this case calcium hydroxide, carbon dioxide,sulfur-dioxide or phosphoric acid, for example, is added to purify thelight syrup. Sparingly soluble calcium salts such as calcium carbonate,sulfate or phosphate remaining in the syrup after filtration thenprecipitate during the evaporation process and may produce hard depositson the surfaces of heat exchangers. This also applies to substances alsopresent in the sugar, such as silica or calcium salts of organic acidssuch as oxalic acid.

Similar statements apply to processes following sugar production, e.g.alcohol production from residues thereof.

The copolymers which can be used according to the invention to preventdeposits are able substantially to suppress the abovementioned depositsso that system stoppages for cleaning, e.g. by boiling out, can beconsiderably reduced. Another essential point in this connection is theconsiderable saving in energy due to prevention of these deposits of lowthermal conductivity.

The amounts of antideposit agent required for the described uses vary,but are from 0.1 to 100 ppm based on the cooling, boiler or processwater used or, for example, syrup.

The products to be used according to the invention are more effective atdispersing salts such as calcium carbonate, sulfate and phosphate and,furthermore, are more compatible with calcium ions than are acrylic acidhomopolymers.

The K values of the copolymers were determined by the method of H.Fikentscher, Cellulosechemie, 13 (1932) 48 to 64 and 71 to 74, inaqueous solution at pH 7, 25° C. and a concentration of the sodium saltof the copolymer of 1% by weight. Percentage data relate to the weightof these substances.

EXAMPLES Preparation of the Copolymers Copolymer 1

In a reactor equipped with condenser, thermometer, feed devices and aninlet and outlet for nitrogen, a solution of 1.9 g of phosphorous acidin 370 g of water is heated to boiling. To this are added during 4 hoursat a constant rate 590 g of acrylic acid, a solution of 63.6 g ofN-vinylimidazole in 100 g of water, a solution of 54 g of2-mercaptoethanol in 50 g of water and a solution of 6.4 g of sodiumpersulfate and 0.6 g of 2,2'-azobis(2-methylpropionamidine)dihydrochloride in 125 g of water, keeping the reaction mixture boilinggently during the introduction. After the addition of monomers andinitiators is complete, the reaction mixture is boiled for 1 hour andthen neutralized to pH 7.5 with 620 g of 50% strength aqueous sodiumhydroxide solution. The residue on drying is 44%, and the K value of thecopolymer is 39.

Copolymer 2

The process described in Example 1 is carried out but using 524 g ofacrylic acid and 127.2 g of N-vinylimidazole and, after completion ofthe copolymerization, neutralizing the aqueous copolymer solution to pH7.3 by adding 540 g of 50% strength aqueous sodium hydroxide solution.The residue on drying is 45%, and the K value of the copolymer is 47.5.

Copolymer 3

1028.5 g of a mixture of 75% by weight isopropanol and 25% by weightwater plus 72 g of 30% strength sodium peroxide are placed in a steelreactor designed for elevated pressure. Nitrogen is passed into thereactor up to a pressure of 3 bar and released again 3 times. Thereactor is then sealed and the contents are heated while stirring to120° C. As soon as this temperature is reached, 508 g of a mixture of75% isopropanol and 25% water, 1758.5 g of acrylic acid and 189.5 g ofN-vinylimidazole and, separately from this, at a constant rate during 8hours, a mixture of 122 g of 30% strength hydrogen peroxide and 250 g ofisopropanol are metered in. The pressure in the reactor is kept constantat 3 bar during the polymerization. After addition of the initiator iscomplete, the rection mixture is heated at 120° C. for 2 hours and thenthe pressure is released, with some of the isopropanol distilling out,and subsequently the remaining isopropanol is removed by steamdistillation. The reaction mixture is then cooled to 50° C. andneutralized by adding 1850 g of 50% strength aqueous sodium hydroxidesolution. The pH of the aqueous solution is 8, the residue on drying is46.5% and the K value of the copolymer is 30.2.

Copolymer 4

1028.5 g of isopropanol and 78.6 g of 30% strength hydrogen peroxide areplaced in the reactor used to prepare copolymer 3, which is then flushedwith 3 bar of nitrogen 3 times and, after the reactor has been sealed,heated to 120° C., resulting in a pressure of 3 bar. As soon as thereactor contents are at 120° C., a mixture of 908 g of isopropanol, 1516g of acrylic acid, 189.5 g of N-vinylimidazole and 189.5 g ofacrylamidomethylpropanesulfonic acid is metered in at a constant rateduring 5 hours and, separately from this, a solution of 133 g of 30%strength hydrogen peroxide in 270 g of isopropanol is metered in at aconstant rate during 6 hours. After the addition of initiator iscomplete, the reaction mixture is heated at 120° C. for 2 hours, thenthe pressure is released and isopropanol is distilled out. The reactionmixture is cooled to 50° C. and neutralized to pH 8.1 with 1950 g of 50%strength aqueous sodium hydroxide solution. The solids content of theaqueous solution is 45.1%, and the K value of the copolymer is 33.2.

Copolymer 5

A solution of 1.9 g of phosphorous acid in 370 g of water is placed inthe reactor used to prepare copolymer 1, and the solution is heated toboiling. Then 573.3 g of acrylic acid and a solution of 106 g of1-vinyl-3-methylimidazolium chloride in 58 g of water, a solution of 54g of 2-mercaptoethanol in 50 g of water and a solution of 6.4 g ofsodium persulfate and 0.64 g of 2,2'-azobis(2-methylpropionamidine)hydrochlorid in 125 g of water are added at constant rate during 4hours. After the addition of initiators and monomers is complete, thereaction mixture is heated at 100° C. for 2 hours, then cooled to 50° C.and neutralized by adding 590 g of 50% strength aqueous sodium hydroxidesolution. The solids content of the aqueous solution is 44.5%. Thecopolymer has a K value of 17.7.

Copolymer 6

A solution of 1.9 g of phosphorous acid in 370 g of distilled water isplaced in the reactor used to prepare copolymer 1 and heated to 95° C.To this are added, at a constant rate during 4 hours, a solution of508.8 g of acrylic acid, 35 g of water and 127.2 g of N-vinylimidazole,a solution of 54 g of mercaptoethanol in 50 g of water and, separatelyfrom this, a solution of 3.2 g of sodium persulfate and 3.2 g of2,2'-azobis(2-methylpropionamidine) hydrochloride in 130 g of water. Theresult is an aqueous solution of a copolymer with a solids content of45.6% and a K value of 20.3.

Copolymer 7

1028.5 g of isopropanol and 78.6 g of 30% strength hydrogen peroxide areplaced in the reactor used to prepare copolymer 3, which is then flushedwith 3 bar of nitrogen 3 times and, after sealing, the contents of thereactor are heated to 120° C. This results in a pressure of 3 bar. Assoon as the temperature is 120° C., a mixture of 1327 g of acrylic acid,529 g of isopropanol, 189.5 g of N-vinylimidazole, 379 g of a 25%strength aqueous acrylamidomethylpropanesulfonic acid solution and 758 gof a 25% strength aqueous acrylamide solution is added at a constantrate during 5 hours, and, separate from this, a solution of 133 g of 30%strength hydrogen peroxide in 271 g of isopropanol is added at aconstant rate in 6.5 hours. After the addition of initiator is complete,the reaction mixture is stirred at 120° C. for 1 hour and then thepressure is slowly released while distilling out isopropanol. Steam isthen passed into the reaction mixture to remove the isopropanol and thisis continued until the temperature has reached 100° C. The reactionmixture is then cooled to 50° C. and neutralized by adding 1200 g of 50%strength aqueous sodium hydroxide solution. The resulting solution has apH of 8 and a solids content of 43.4%. The K value of the copolymer is24.5.

Copolymer 8

1028 g of a mixture of 75% sec-butanol and 25% water and 78.6 g of 30%strength hydrogen peroxide are placed in the reactor used to preparecopolymer 3, which is then flushed with 3 bar of nitrogen 3 times and,after sealing, the contents of the reactor are heated to 120° C. As soonas this temperature is reached, a solution of 908 g of sec-butanol,1326.5 g of acrylic acid, 189.5 g of N-vinylimidazole and 379 g ofacrylamidomethylpropanesulfonic acid is metered in during 5 hours, and asolution of 133 g of 30% strength hydrogen peroxide in 270 g of amixture of 70% by weight sec-butanol and 30% by weight of water ismetered in at a constant rate during 6.5 hours. After the addition ofinitiator is complete, the reaction mixture is heated at 120° C. for 2hours and the pressure is then slowly released, during which thesec-butanol distils out. The remaining sec-butanol is removed by passingin steam until the internal temperature is 100° C. The reactor contentsare then cooled to 50° C. and neutralized by adding 1500 g of 50%strength aqueous sodium hydroxide solution. The resulting copolymersolution has a pH of 7 and a solids content of 51.6%. The K value of thecopolymer is 22.6.

Copolymer 9

1028.5 g of a mixture of 75% isopropanol and 25% water and 78.6 g of 30%strength hydrogen peroxide are introduced into the reactor used toprepare copolymer 3, which is then flushed with 3 bar of nitrogen 3times and, after sealing, heated to 130° C. At this temperature, amixture of 1706 g of acrylic acid, 189.5 g of N-vinylimidazole and 961 gof a mixture of 75% isopropanol and 25% water is added during 6 hoursand, separately from this, a solution of 133 g of 30% strength hydrogenperoxide and 251 g of a mixture of 75% isopropanol and 25% water isadded during 8 hours. After the addition of initiator is complete, thereaction mixture is stirred at 130° C. for 2 hours and the pressure isthen cautiously released while isopropanol distils out. After thepressure has reached atmospheric, steam is passed in to remove theremaining isopropanol. The reaction mixture is then cooled to 50° C. andneutralized to pH 8.1 with 1890 g of a 50% strength aqueous sodiumhydroxide solution. The resulting copolymer solution has a solidscontent of 51.2%. The K value of the copolymer is 26.4.

Copolymer 10 (Comparison)

Commercial acrylic acid homopolymer with a K value of 30.

The copolymers described above were subjected to the following tests oftheir suitability for water treatment:

CaSO₄ Test

500 ml of a saturated CaSO₄ solution is concentrated to 200 g in an ovenat 200° C. The mixture is left to stand overnight and then filteredthrough a membrane filter (0.45 μm).

50 ml of the filtrate is titrated against an aqueous 0.2M solution ofNa₂ H₂ EDTA (EDTA=ethylenediaminetetraacetic acid) and the proportion ofCa still in solution is determined. The inhibition on addition of 1 ppmpolymer is calculated by comparison with a blank containing no polymer.##EQU1##

CaCO₃ Test

An aqueous test solution is prepared from components A and B:

A=3.36 g of NaHCO₃ per liter

B=1.58 g of CaCl₂ ·2H₂ O per liter and 0.88 g of MgSO₄ per liter

100 ml of each of the above solutions are pipetted into a 250 ml flask,5 ppm dispersing agent is added, and the flask is stoppered and storedat 86° C. for 16 hours. After the solution has been cooled to roomtemperature and filtered it is titrated against a 0.2M solution of Na₂H₂ EDTA to determine the proportion of dissolved Ca. ##EQU2##

Ca₃ (PO₄)₂ Test

100 ml of a solution with the following concentrations are prepared:

1.095 g/l CaCl₂ ·6H₂ O

0.019 g/l Na₂ HPO₄ ·2H₂ O

2 ppm polymer

The pH is adjusted to 8.6 with a borax buffer. The solution is thenstirred at 70° C. for 3 hours and left to stand for 24 hours. After thistime the light transmission (LT, white light) is measured with aphotometer. 100% LT on the photometer is set with distilled waterbeforehand. ##EQU3##

Ca Ion Compatibility

200 ml of a solution with the following composition are prepared:

1.565 g of CaCl₂ ·6H₂ O per liter

3 g of KCl per liter

45 ppm polymer

The pH is adjusted to 9 with NaOH and the solution is then boiled for 30minutes. The boiled solution is then made up to 200 ml with distilledwater and the light transmission (LT) is measured (LT for distilledwater=100%). The LT is directly related to the compatibility of theproduct with Ca ions.

The results of the tests are shown in the table.

    ______________________________________                                                                             LT %                                            Co-                           (poly-                                          poly-                                                                              K                        mer                                             mer  val-   % Inhibition      tur-                                     Ex.      No.    ue     CaSO.sub.4                                                                          CaCO.sub.3                                                                          Ca.sub.3 (PO.sub.4).sub.2                                                             bidity)                            ______________________________________                                        1        1      39     35    53    58      83                                 2        2      47.5   42    45    72      92                                 3        3      30     47    49    66      97                                 4        4      33     27    43    63      91                                 5        5      17.7   39    55    73      --                                 6        6      20.3   48    39    70      --                                 7        7      24.5   41    23    78      --                                 8        8      22.6   51    30    72      100                                9        9      26.4   39    56    77      97                                 Comparison      30     29    45    61      <60                                Example 10                                                                    (acrylic acid                                                                 hompolymer)                                                                   ______________________________________                                    

It is evident from the table that copolymers 2, 3, 5 and 9 are moreeffective than the acrylic acid homopolymer in all the tests. Thesecopolymers are particularly suitable for treating those water-conveyingsystems which have high Ca ion concentrations, e.g. in the evaporationof syrup.

Copolymers 1, 4, 6, 7 and 8 are better than the acrylic acid homopolymerin 3 of 4 tests.

We claim:
 1. A process for reducing the deposition of scale and sludgein a water-conveying system, which comprises adding to said system anamount effective to reduce deposition of scale and sludge of awater-soluble copolymer of(a) 95 to 70% by weight of acrylic acid,methacrylic acid and/or maleic acid or salts thereof and (b) 5 to 30% byweight of N-vinylimidazole or the salt or the product of quaternizationthereof, which has a Fikentscher-K value of from 10 to 50 determined in1% strength aqueous solution at pH and 25° C.
 2. A process as claimed inclaim 1, wherein the copolymer contains copolymerized therein(c) up to20% by weight of another monoethylenically unsaturated monomer which canbe copolymerized with monomers (a) and (b).
 3. A process as claimed inclaim 1, wherein the copolymer is used in amounts of from 0.1 to 100 ppmbased on the aqueous medium to be treated.